<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>后代选择器</title>

  <style>
    div span {
      color: red;
    }
  </style>

</head>
<body>

  <span> span 标签</span>
  <div>
    <span>这是 div 的儿子 span</span >
  </div>
</body>
</html>